GIS Romania

8/2/2019 GIS Romania

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Generalitati

Introducere

Sistemul GIS (Geographic Information System) este un sistem informational, bazat pe

computer, ce foloseste reprezentarea si analiza digitala a elementelor geografice cesunt prezente la suprafata terestra si a evenimentelor (atribute nonspatiale legate degeografia studiata) ce au loc pe acestea. Intelesul reprezentarilor digitale este conversiaformei analogice (linie continua) intr-una digitala.Fiecare obiect prezent pe Pamant poate fi geo-referentiat este elementul fundamentalpentru asocierea oricarei baze de date GIS. Termenul baza de date este o colectie deinformatii despre lucruri si asocierea acestora unora cu altele, geo referentierea serefera la locatia unui nivel sau acoperirea in spatiu definit de un sistem referential decoordonate.

Definirea GIS

Un GIS este un sistem informatic proiectat sa lucreze cu date referentiate de catrecoordonate spatiale/geografice. Cu alte cuvinte, GIS este atat un sistem de baze dedate cu capacitati specifice pentru datele referentiate spatial cat si un set de operatiicare lucreaza cu aceste date. El este deseori considerat ca o harta de nivel inalt.

Tehnologia GIS integreaza operatii comune pentru baze de date cum ar fi interogari sianaliza statistica, cu vizualizarea unica si beneficiile analizelor geografice oferite decatre harti. Aceste caracteristici deosebesc GIS de alte sisteme informatice si fac caacesta sa aiba valoare pentru o gama larga de public si de intreprinderi private pentruexplicarea unor evenimente, previziunea unor rezultate si planificarea strategiilor.

Un sistem GIS este un sistem bazat pe computer care este utilizat pentru analiza sireproducerea elementelor prezente pe suprafata Pamantului si a evenimentelor care auloc pe aceste elemente. Pentru sublinierea rolului GIS trebuie amintit faptul ca 70% dindate au referinte geografice.

Un sistem tipic GIS poate fi inteles cu ajutorul definitiilor urmatoare:

- un sistem GIS este un instrument bazat pe computer pentru crearea de harti sianaliza lucrurilor care exista si a evenimentelor care au loc pe Pamant.

- Burrough definea GIS in 1986 ca Set de instrumente pentru colectarea,

stocarea, apelarea, transformarea si afisarea datelor spatiale de la lumea realapentru un set de obiective particulare.- Arnoff definea GIS in 1989 ca un sistem bazat pe computer care furnizeaza

patru seturi de functiuni pentru manipularea datelor geo-referentiate:- Intrarea datelor;- Managementul datelor (stocarea si apelarea datelor);- Manipularea si analiza;- Iesirea datelor.


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GIS poate fi privit si ca un instrument pentru asistarea in luarea deciziilor simanagementul atributelor care cer analiza spatiala.

Factori care au contribuit la dezvoltarea GIS:- revolutia din domeniul tehnologiei informatiei;- tehnologia calculatoarelor;- captarea de imagini din satelit;- GPS;

- Tehnologia comunicatiei;- Scaderea rapida a hardware-ului calculatoarelor, si in acelasi timp, si cresterea

exponentiala a vitezei de operare a calculatoarelor;- Imbunatatirea functionalitatii software si interfetelor cu utilizatorul;

Avantajele GIS

GIS este un instrument efectiv pentru implementarea si monitorizarea infrastructurii unuioras. Astfel un GIS poate avea urmatoarele avantaje:

a) Planificarea proiectelorAvantajele GIS sunt deseori gasite in planificarea in detaliu a proiectului care are multecomponente spatiale, unde analiza problemei este o necesitate inainte de incepereaproiectului. Generarea hartilor tematice este posibila pe baza uneia sau mai multor hartide baza, de exemplu: generarea unei harti privind utilizarea terenurilor pe bazacompozitie solului, a vegetatiei si a topografiei. Combinatia unica a unor elementeconcrete faciliteaza crearea unei astfel de harti tematice. Prin intermediul diferitelormodule, in GIS este posibila calcularea suprafetelor, lungimilor si a distantelor.

b) Luarea deciziilorZicala mai buna informare duce la decizii mai bune este un adevar pentru GIS cat sipentru alte sisteme informatice. Un GIS nu este un sistem automat de luare a deciziilordar este un instrument pentru interogarea, analiza si dispunerea datelor pe harta utilizatla sprijinirea procesului de luare a deciziilor. Tehnologia GIS poate fi utilizata laasistenta in procese cum ar fi prezentarea informatiilor la investigatii, sprijin inrezolvarea disputelor teritoriale si dispunerea pilonilor intr-o zona cu acces dificil.

c) Analiza vizualaDTM (Digital Terrain Modeling) este un utilitar important al GIS. Folosind modelareaDTM/3D, terenul poate fi mai bine vizualizat, acest lucru conducand la o intelegere maibuna a relatiilor din cadrul acelui teren. Astfel utilizand GIS multe calcule si modelarihidrologice devin mult mei usoare, de exemplu volumul apelor si lacurilor, volumuleroziunii solului, cantitatea de pamant care trebuie excavata (pentru canale, drumuri,baraje). GIS nu este folosit numai in domeniile mentionate pana acum ca exemple, elpoate fi folosit si in stiinte sociale (analiza distributie populatiei, probleme administrativeetc.).

d) Imbunatatirea integrarii organizationaleMulte organizatii care au implementat un sistem GIS au gasit, ca unul din principalelebeneficii, este imbunatatirea managementului organizatiei si resurselor. Deoarece GIS


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are calitatea de a seturi de date impreuna prin intermediul geografiei, el faciliteazacomunicarea si partajarea informatiilor intre departamente. Prin crearea unei baze dedate partajate un departament poate beneficia de munca altui departament, datele pot ficolectate o singura data si pot folosite de mai multe ori.

Componentele GIS

Sistemele GIS sunt compuse din urmatoarele cinci componente fundamentale:

- hardware;- software;- date;- oameni;- metode.

Hardware

Acesta consta din sistemul unui computer pe care va rula un sofware GIS. Se poatealege un sistem hardware incepand cu un calculator personal cu un procesor de 300MHz si terminand cu un super computer care capacitatea de ordinul TeraFLOPS.

Software

Software-ul GIS furnizeaza functiile si instrumentele necesare stocarii, analizei si afisariiinformatiilor geografice. Cateva exemple de software GIS sunt urmatoarele: MapInfo,ARC/Info, AutoCAD Map etc. Software-ul necesar este dictat de catre specificulaplicatiei.

Date

Datele geografice si datele tabelare pot fi colectate in cadrul firmei sau cumparate de lafurnizorii de date comerciale. Hartile digitale formeaza intrarile de date de baza pentruGIS. Un sistem GIS va integra datele spatiale cu alte resurse de date si pot chiar utilizaun DBMS, folosit de majoritatea organizatiilor sa intretina datele lor specifice, pentrugestionarea datelor spatiale.

Oameni

Utilizatorii GIS sunt de la specialistii care proiecteaza si intretin sisteme pana la cei carefolosesc aceste sisteme sa isi faca treaba de zi cu zi. Utilizatorii de sisteme GIS pot foimpartiti in doua mari categorii. Operatori CAD/GIS a caror sarcina consta invectorizarea obiectelor pe harti. Utilizarea acestor date vectorizate pentru realizarea deinterogari, analize si a altor aplicatii sunt responsabilitatea inginerilor/utilizatorilor GIS.


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Metode

Deasupra tuturor, pentru ca un sistem GIS sa fie un succes, acesta trebuie sa operezedupa un plan bine proiectat si dupa un set de reguli, acestea sunt modele si practici deoperare unice pentru fiecare organizatie. Exista diferite tehnici folosite pentru crearea

de harti si pentru utilizarea ulterioara a acestora. Hartile pot fi create in mod automatsau manual folosind imagini scanate.

Aplicatii GIS

Cartografierea computerizata si analiza spatiala au fost dezvoltate simultan in catevadomenii. Starea actuala nu ar fi fost atinsa daca nu ar fi fost o interactiune stransa intrediferite domenii precum: retelele de utilitati, cartografierea cadastrala, cartografiereatopografica, cartografierea tematica, procesarea de imagini, informatica, planificareaurbana si rurala, stiintele pamantului si geografie. Tehnologia GIS devine rapid uninstrument standard pentru managementul resurselor naturale. Utilizarea efectiva a unui

mare volum de date spatiale este dependenta de existenta unui sistem eficient demanevrare a datelo geografice si de procesare a acestora pentru transformarea datelorin informatii utilizabile. Tehnologia GIS este utlizata sa asiste managerii in luareadeciziilor prin indicarea diferitelor alternative in planificarea dezvoltarii si a conservarii siprin modelarea rezultatelor potentiale ale diferitelor scenarii. Trebuie remarcat faptul caorice proces incepe si se termina cu lumea reala. Datele sunt colectate din lumea reala.

Principalele domenii de aplicare

- Diferite activitati de planificare:- Planificare urbana, constructia de locuinte, planificarea transporturilor,

conservarea arhitecturala, design urban, planificarea terenurilor;- Aplicatii bazate pe reteaua de strazi: definirea si organizarea rutelor vehiculelor,aplicatii pentru localizare si interventie in caz de dezastre;

- Aplicatii bazate pe resursele naturale: managementul mediului si analizaimpactului asupra mediului, distributia si exploatarea resurselor naturale;

- Analize: amplasamentele fabricilor de materiale toxice sau cu grad ridicat de risc,studierea habitatelor animalelor si rutelor de migrare;

- Managementul terenurilor: zonarea (delimitarea unor anumitor zone, in cadruloraselor sau a altor entitati organizatorice, ce au anumite caracteristici), achizitiade terenuri, analiza impactului de mediu, managementul si intretinerea calitatiinaturale a terenurilor;

- Managementul utilitatilor: localizarea conductelor si a cablurilor subterane pentruactivitati de intretinere, planificare si interventie


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Elemente fundamentale despre GIS

Harti. Concepte, elemente si proprtietati.O harta reprezinta elemente geografice sau alte fenomene spatiale prin transmitereainformatiilor despre localizarea acestora si atributele lor. Informatia de localizare descrie

pozitia unor elemente geografice particulare pe suprafata Pamantului, precum si relatiilespatiale dintra aceste elemente, cum ar fi calea cea mai scurta de la o statie depompieri la o biblioteca etc. Informatiile atribute descriu caracteristicile elementelorgeografice reprezentate, cum ar fi tipul elementului, numele sau numarul acestuia siinformatii cantitative, cum ar fi aria sau lungimea.Astfel obiectivele principale ale cartografierii sunt sa ofere:

- descrierea fenomenelor geografice;- informatii spatiale si non-spatiale;- elemente ale hartilor cum ar fi puncte, linii si poligoane.

Elementele hartilor

Informatia de localizare este in mod uzual reprezentata de catre puncte pentru diferiteelemente, cum ar fi un dulap de echipamente sau un post de telefonie, linii pentruelemente cum ar fi conducte, cabluri electrice sau linii de contur, si arii pentru elementecum ar fi lacuri, teritorii si zone.

PuncteUn punct reprezinta o singura locatie. Acesta defineste un obiect pe harta care esteprea mic pentru a putea fi redat printr-o linie sau o arie. Pentru redarea unui astfel depunct se utilizeaza etichete.LiniiO linie este un set de coordonate conectate si ordonate ce reprezinta o forma liniara aunui obiect de pe harta, acesta fiind prea subtire pentru a putea fi reprezentat printr-oarie (cum ar fi un drum sau un element al hartii care nu are grosime, precum liniile decontur).AriileO arie este o figura inchisa ale carei limite cuprind o arie omogena, cum ar fi state,terenuri sau lacuri.

Caracteristicile hartilorPe langa localizarea elementelor si a atributelor, exista si alte caracteristici tehnice caredefinesc hartile si utilizarea acestora:

- scara hartii;- acuratetea hartii;- acoperirea hartii;- acoperirea bazei de date.

ScaraPentru a reda o portiune din suprafata Pamantului pe o harta, scara trebuie sa fieajustata suficient pentru a acoperi obiectivul. Scara hartii sau acoperirea redusa esteexpesia unui raport. Unitatea din stanga reprezinta distanta pe harta iar numarul din


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dreapta indica distanta pe suprafata terestra (1:60.000.000 = 1cm pe harta arecorespondentul 60 milioane cm pe suprafata Pamantului). Scara unei harti arata cat demult poate fi redusa o arie data. Pentru aceeasi dimensiune a hartii, elementele pe oharta la scara mica (1:10.000.000) vor fi mai mici decat aceleasi elemente pe o harta lascara mare (1:1.000). O harta cu mai putine detalii este la o scara mai mica decat una

cu mai multe detalii. Cartografii impart hartile, dupa scara, in trei categorii diferite:- harti la scara mica care au scara mai mica decat 1:1.000.000 si sunt folositepentru arii foarte extinse unde nu sunt necesare foarte multe detalii;

- harti la scara medie au scara cuprinsa intre 1:75.000 si 1:1.000.000;- harti la scara mare au scara mai mare decat 1:75.000 si sunt folosite in

aplicatiile in care este nevoie de multe detalii.

So each scale represents a different tradeoff. With a small-scale map, you'll be able to show a large areawithout much detail. On a large-scale map, you'll be able to show a lot of detail but not for a large area.The small-scale map can show a large area because it reduces the area so much that the large-scalemap can only show a portion of one street, but in such detail that you can see shapes of the houses.

To convert this statement to a representative fraction, the units of measure on both the sides beingcompared must be the same. For this example, both measurements will be in meters.

To do this:

1. Convert 1.6 inches into meters

1.6 inches x 0.0254 meters/inch = 0.04 meters

2. Let us suppose that

0.04 units on the map = 10,000 units on the ground

Then, you can now state the scale as a representative fraction (RF): 0.04:10,000

Though it is a valid statement of scale, most cartographers may find it clumsy. Traditionally, the firstnumber in the representative fraction is made equal to 1:

0.04 / 0.04 = 1 units on the map = 10,000 / 0.04 units on the ground

1 unit on the map = 250,000 units on the ground

Scale in Digital MapsWith digital maps, the traditional concept of scale in terms of distance does not apply because digitalmaps do not remain fixed in size. They can be displayed or plotted at any possible magnification. Yet westill speak of the scale of a digital map.

In digital mapping, the term scale is used to indicate the scale of the materials from which the map wasmade. For example, if a digital map is said to have a scale of 1:100,000, it was made from a 1:100,000-scale paper map.

However, a digital map's scale still allows you to make some educated guesses about its contentsbecause, generally, digital maps retain the same accuracy and characteristics as their source maps. So it


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is still true that a large-scale digital map will usually be more accurate and less general than a small-scaledigital map.

Because the display size of a computer-based map is not fixed, users are often tempted to blow up mapsto very large sizes. For example, a 1:100,000-scale map can easily be plotted at a size of 1:24,000 oreven 1:2,000-but it usually is not a good idea to do so. It encourages the user to make measurementsthat the underlying data does not support. You cannot measure positions to the nearest foot if your map isonly accurate to the nearest mile. You will end up looking for information that does not exist.Map ResolutionMap resolution refers to how accurately the location and shape of map features can be depicted for agiven map scale. Scale affects resolution. In a larger-scale map, the resolution of features more closelymatches real-world features because the extent of reduction from ground to map is less. As map scaledecrease, the map resolution diminishes because features must be smoothed and simplified, or notshown at all.

Map AccuracyMany factors besides resolution, influence how accurately features can be depicted, including the qualityof source data, the map scale, your drafting skill and the width of lines drawn on the ground. A finedrafting pen will draw line's 1/100 of an inch wide. Such a line represents a corridor on the ground, whichis almost 53 feet wide.

In addition to this, human drafting errors will occur and can be compounded by the quality of your sourcemaps and materials. A map accurate for one purpose is often inaccurate for others since accuracy isdetermined by the needs of the project as much as it is by the map itself.

Some measurements of a map's accuracy are discussed below.

Absolute accuracy of a map refers to the relationship between a geographic position on a map (astreet corner, for instance) and its real-world position measured on the surface of the earth.Absolute accuracy is primarily important for complex data requirements such as those forsurveying and engineering-based applications.

Relative accuracy refers to the displacement between two points on a map (both distance andangle), compared to the displacement of those same points in the real world. Relative accuracy is

often more important and easier to obtain than absolute accuracy because users rarely need toknow absolute positions. More often, they need to find a position relative to some knownlandmark, which is what relative accuracy provides. Users with simple data requirementsgenerally need only relative accuracy.

Attribute accuracy refers to the precision of the attribute database linked to the map's features.For example, if the map shows road classifications, are they correct? If it shows street addresses,how accurate are they? Attribute accuracy is most important to users with complex datarequirements.

A map's Currency refers to how up-to-date it is. Currency is usually expressed in terms of arevision date, but this information is not always easy to find.

A map is Complete if it includes all the features a user would expect it to contain. For example,does a street map contain all the streets? Completeness and currency usually are relatedbecause a map becomes less complete as it gets older.

The most important issue to remember about map accuracy is that the more accurate the map, the moreit costs in time and money to develop. For example, digital maps with coordinate accuracy of about 100feet can be purchased inexpensively. If 1-foot accuracy is required, a custom survey is often the only wayto get it, which drives up data-acquisition costs by many orders of magnitude and can significantly delayproject implementation - by months or even years.

Therefore, too much accuracy can be as detrimental to the success of a GIS project as too little. Ratherthan focusing on the project's benefits, a sponsoring organization may focus on the costs that result from


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a level of accuracy not justified for the project. Project support inevitably erodes when its originalobjectives are forgotten in a flurry of cost analyses.

A far better strategy is to start the project with whatever data is readily available and sufficient to supportinitial objectives. Once the GIS is up and running, producing useful results, project scope can beexpanded. The quality of its data can be improved as required.

Even though no maps are entirely accurate, they are still useful for decision-making and analysis. However, it is important to consider map accuracy to ensure that your data is not used inappropriately.

Any number of factors can cause error. Note these sources can have at cumulative effect.

E = f(f) + f(1) + f(e) + f(d) + f(a) + f(m) + f(rms) + f(mp) + u

Where,

f = flattening the round Earth onto a two - dimensional surface (transformation from spherical toplanar geometry)I = accurately measuring location on Earth (correct project and datum information)c = cartographic interpretation (correct interpretation of features)

d = drafting error (accuracy in tracing of features and width of drafting pen)a = analog to digital conversion (digitizing board calibration)m = media stability (warping and stretching, folding. Wrinkling of map)p = digitizing processor error (accuracy of cursor placement)rms = Root Mean Square (registration accuracy of ties)mp = machine precision (coordinate rounding by computer in storing and transforming)u = additional unexplained source error

Map ExtentThe aerial extent of map is the area on the Earth's surface represented on the map. It is the limit of thearea covered, usually defined by rectangle just large enough to include all mapped features. The size ofthe study area depends on the map scale. The smaller the scale the larger the area covered.

Database ExtentA critical first step in building a geographic database is defining its extent. The aerial extent of a databaseis the limit of the area of interest for your GIS project. This usually includes the areas directly affected byyour organization's responsibility (such as assigned administrative units) as well as surrounding areasthat either influence or are influenced by relevant activities in the administrative area.

Data AutomationMap features are logically organized into a set of layers or themes of information. A base map can beorganized into layers such as streams, soils, wells or boundaries. Map data, regardless of how a spatialdatabase will be applied, is collected, automated and updated as series of adjacent map sheets or aerialphotograph. Here each sheet is mounted on the digitizer and digitized, one sheet at a time. In order to beable to combine these smaller sheets into larger units or study areas, the co-ordinates of coverage must

be transformed into a single common co-ordinate system. Once in a common co-ordinate system,attributes are associated with features. Then as needed map sheets for layer are edge matched andjoined into a single coverage for your study area.

Types of Information in a Digital MapAny digital map is capable of storing much more information than a paper map of the same area, but it'sgenerally not clear at first glance just what sort of information the map includes. For example, moreinformation is usually available in a digital map than what you see on-screen. And evaluating a given dataset simply by looking at the screen can be difficult: What part of the image is contained in the data andwhat part is created by the GIS program's interpretation of the data? You must understand the types of


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data in your map so you can use it appropriately.

Three general types of information can be included in digital maps:

Geographic information, which provides the position and shapes of specific geographic features.

Attribute information, which provides additional non-graphic information about each feature.

Display information, which describes how the features will appear on the screen.

Some digital maps do not contain all three types of information. For example, raster maps usually do notinclude attribute information, and many vector data sources do not include display information.

Geographic InformationThe geographic information in a digital map provides the position and shape of each map feature. Forexample, a road map's geographic information is the location of each road on the map.

In a vector map, a feature's position is normally expressed as sets of X, Y pairs or X, Y, Z triples, usingthe coordinate system defined for the map (see the discussion of coordinate systems, below). Most vectorgeographic information systems support three fundamental geometric objects:

Point:A single pair of coordinates. Line:Two or more points in a specific sequence.

Polygon:An area enclosed by a line.

Some systems also support more complex entities, such as regions, circles, ellipses, arcs, and curves.

Attribute InformationAttribute data describes specific map features but is not inherently graphic. For example, an attributeassociated with a road might be its name or the date it was last paved. Attributes are often stored indatabase files kept separately from the graphic portion of the map. Attributes pertain only to vector maps;they are seldom associated with raster images.

GIS software packages maintain internal links tying each graphical map entity to its attribute information.

The nature of these links varies widely across systems. In some, the link is implicit, and the user has nocontrol over it. Other systems have explicit links that the user can modify. Links in these systems take theform of database keys. Each map feature has a key value stored with it; the key identifies the specificdatabase record that contains the feature's attribute information.

Display InformationThe display information in a digital-map data set describes how the map is to be displayed or plotted.Common display information includes feature colours, line widths and line types (solid, dashed, dotted,single, or double); how the names of roads and other features are shown on the map; and whether or notlakes, parks, or other area features are colour coded.

However, many users do not consider the quality of display information when they evaluate a data set.Yet map display strongly affects the information you and your audience can obtain from the map - no

matter how simple or complex the project. A technically flawless, but unattractive or hard-to-read map willnot achieve the goal of conveying information easily to the user.

Cartographic AppealClearly, how a map looks - especially if it is being used in a presentation - determines its effectiveness.Appropriate color choices, linetypes, and so on add the professional look you want and make the mapeasier to interpret. Since display information often is not included in the source data set or is filtered outby conversion software, you may need to add it yourself or purchase the map from a vendor who does itfor you. Map display information should convey the meaning of its underlying attribute data.


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Various enhancements will increase a map's usefulness and cartographic appeal.

Feature Colors and Linetypes. Colors and line representations should be chosen to make themap's meaning clear. For example, using double-line roads can be quite helpful. Many GIS datasets only include road centerline information. Actual road width is not given. So maps with

centerlines only can look like spider webs, which is visually unappealing. Some software andconversion systems can draw roads as double lines, with distance between lines varyingaccording to road type. Centerlines can be included, if necessary. Double-line maps areappropriate for detailed studies of small areas, such as subdivisions, or maps where right-of-wayinformation is important.

Naming Roads. Naming, or labeling, roads are important for proper map interpretation. Thisinformation should be legible, positioned in the center of the road or offset from the center, anddrawn at intervals suited to the scale of the final map or its purpose.

Landmark Symbols. A good set of symbols should be used to indicate landmarks, such ashospitals, schools, churches, and cemeteries. The symbols should be sized appropriately inrelation to map scale.

Polygon Fills. Polygon features, such as lakes or parks, should be filled with an appropriate coloror hatch pattern.

Zoom Layer Control. If the GIS software platform permits, map layers should be set up so thatdetailed, high-density information only appears when the user zooms in for a close-up of part ofthe map. For example, when a large area is displayed, only the major roads should appear; for asmaller area, both major and minor roads should appear.

LayeringMost GIS software has a system of layers, which can be used to divide a large map into manageablepieces. For example, all roads could be on one layer and all hydrographic features on another. Majorlayers can be further classified into sub-layers, such as different types of roads - highways, city streets,and so on. Layer names are particularly important in CAD-based mapping and GIS programs, which haveexcellent tools for handling them.

Some digital maps are layered according to the numeric feature-classification codes found in their source

data sets. For example, a major road might be on the 170-201 layer. However, this type of system is notvery useful. A well-thought-out layering scheme can make any data set much easier to use because itallows the user to control the features with which you want to work. A good layering standard has layernames that are mnemonic (suggest their meanings) and hierarchical (have a structured classif icationscheme that makes it easy to choose general or specific classes).

For example, a map could have its roads on a layer called RD, its railroads on a layer called RR, its roadbridges on a layer called RD-BRIDGE, and its railroad bridges on a layer called RR-BRIDGE. Thisscheme is mnemonic because it is easy to tell a layer's contents from its name, and it's hierarchicalbecause the user can easily select all the roads, railroads, bridges, road bridges, or railroad bridges.

Maps and Map Analysis

Automated MappingComputer Aided Mapping has its limitations. Goal of GIS is not only to prepare a good map but alsoperform map analysis. Maps are the main source of data for GIS. GIS, though an accurate mapping tool,requires error management.

MAP is a representation on a medium of a selected material or abstract material in relation to the surfaceof the earth (defined by Cartographic association). Maps originated from mathematics. The term Map isoften used in mathematics to convey the motion of transferring the information from one form to another

just as Cartographers transfer information from the surface of the earth to a sheet of paper. Map is usedin a loose fashion to refer to any manual display of information particularly if it is abstract, generalised or


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schematic.

Process involved in the production of Maps:

Selection of few features of the real world.

Classification of selected features in to groups eg. Railway in to different lines. Classification

depends upon the purpose. Simplification of jaggered lines like the coast lines.

Exaggeration of features.

Symbolisation to represent different classes of features.

Drawing Digitization of Maps.Maps can be broadly classified in to two groups:

1. Topographical maps2. Thematic maps

Topographical MapsIt is a reference map showing the outline of selected man-made and natural features of the earth. It oftenacts as a frame for other features Topography refers to the shape of surface represented by contours orshading. It also shows lands, railway and other prominent features.

Thematic mapsThematic maps are an important source of GIS information. These are tools to communicategeographical concepts such as Density of population, Climate, movement of goods and people, land useetc. It has many classifications.

Geographical Data Sets

Geographic Data Types

Although the two terms, data and information, are often used indiscriminately, they both have a specific

meaning. Data can be described as different observations, which are collected and stored. Information isthat data, which is useful in answering queries or solving a problem. Digitizing a large number of mapsprovides a large amount of data after hours of painstaking works, but the data can only render usefulinformation if it is used in analysis.

Spatial and Non-spatial dataGeographic data are organised in a geographic database. This database can be considered as acollection of spatially referenced data that acts as a model of reality. There are two important componentsof this geographic database: its geographic position and its attributes or properties. In other words, spatial


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data (where is it?) and attribute data (what is it?)

Attribute DataThe attributes refer to the properties of spatial entities. They are often referred to as non-spatial datasince they do not in themselves represent location information.

District Name Area Population

Noida 395 sq. Km. 6,75,341

Ghaziabad 385 sq. Km. 2,57,086

Mirzapur 119 sq. Km. 1,72,952

Spatial dataGeographic position refers to the fact that each feature has a location that must be specified in a uniqueway. To specify the position in an absolute way a coordinate system is used. For small areas, thesimplest coordinate system is the regular square grid. For larger areas, certain approved cartographicprojections are commonly used. Internationally there are many different coordinate systems in use.

Geographic object can be shown by FOUR type of representation viz., points, lines, areas, andcontinuous surfaces.

Point DataPoints are the simplest type of spatial data. They are-zero dimensional objects with only a position inspace but no length.

Line DataLines (also termed segments or arcs) are one-dimensional spatial objects. Besides having a position inspace, they also have a length.

Area Data

Areas (also termed polygons) are two-dimensional spatial objects with not only a position in space and alength but also a width (in other words they have an area).

Continuous SurfaceContinuous surfaces are three-dimensional spatial objects with not only a position in space, a length anda width, but also a depth or height (in other words they have a volume). These spatial objects have notbeen discussed further because most GIS do not include real volumetric spatial data.

Geographic Data -- Linkages and MatchingLinkagesA GIS typically links different sets. Suppose you want to know the mortality rate to cancer among childrenunder 10 years of age in each country. If you have one file that contains the number of children in this age

group, and another that contains the mortality rate from cancer, you must first combine or link the twodata files. Once this is done, you can divide one figure by the other to obtain the desired answer.


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Exact MatchingExact matching occurs when you have information in one computer file about many geographic features(e.g., towns) and additional information in another file about the same set of features. The operation tobring them together is easily achieved by using a key common to both files -- in this case, the town name.Thus, the record in each file with the same town name is extracted, and the two are joined and stored inanother file.

Name Populaiton

A 4038

B 7030

C 10777

D 5798

E 5606

Name Avg. housing Cost

A 30,500

B 22,000

C 100,000

D 24,000

E 24,000

Name Population Avg. Housing Cost

A 4038 30,500

B 7030 22,000

C 10777 100,100

D 5798 24,000

E 5606 24,000

Hierarchical MatchingSome types of information, however, are collected in more detail and less frequently than other types ofinformation. For example, financial and unemployment data covering a large area are collected quitefrequently. On the other hand, population data are collected in small areas but at less frequent intervals. Ifthe smaller areas nest (i.e., fit exactly) within the larger ones, then the way to make the data match of the


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same area is to use hierarchical matching -- add the data for the small areas together until the groupedareas match the bigger ones and then match them exactly.

The hierarchical structure illustrated in the chart shows that this city is composed of several tracts. Toobtain meaningful values for the city, the tract values must be added together.

Tract Town Population

101 P 60,000

102 Q 45,000

103 R 35,000

104 S 36,000

105 T 57,000

106 Nakkhu 25,000

107 Kupondole 58,000

Tract 101

Tract 102

Tract 103

Tract 104

Tract 105

Tract 107

Tract 106

Fuzzy MatchingOn many occasions, the boundaries of the smaller areas do not match those of the larger ones. Thisoccurs often while dealing with environmental data. For example, crop boundaries, usually defined byfield edges, rarely match the boundaries between the soil types. If you want to determine the mostproductive soil for a particular crop, you need to overlay the two sets and compute crop productivity foreach and every soil type. In principle, this is like laying one map over another and noting thecombinations of soil and productivity.

A GIS can carry out all these operations because it uses geography, as a common key between the datasets. Information is linked only if it relates to the same geographical area.

Why is data linkage so important? Consider a situation where you have two data sets for a given area,such as yearly income by county and average cost of housing for the same area. Each data might beanalysed and/or mapped individually. Alternatively, they may be combined. With two data sets, only onevalid combination exists. Even if your data sets may be meaningful for a single query you will still be ableto answer many more questions than if the data sets were kept separate. By bringing them together, youadd value to the database. To do this, you need GIS.


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Figure 2

Principal Functions of GIS

Data Capture

Data used in GIS often come from many types, and are stored in different ways. A GIS provides tools anda method for the integration of different data into a format to be compared and analysed. Data sourcesare mainly obtained from manual digitization and scanning of aerial photographs, paper maps, andexisting digital data sets. Remote-sensing satellite imagery and GPS are promising data input sources forGIS.

Database Management and Update

After data are collected and integrated, the GIS must provide facilities, which can store and maintain data.Effective data management has many definitions but should include all of the following aspects: datasecurity, data integrity, data storage and retrieval, and data maintenance abilities.

Geographic Analysis

Data integration and conversion are only a part of the input phase of GIS. What is required next is theability to interpret and to analyze the collected information quantitatively and qualitatively. For example,satellite image can assist an agricultural scientist to project crop yield per hectare for a particular region.For the same region, the scientist also has the rainfall data for the past six months collected throughweather station observations. The scientists also have a map of the soils for the region which showsfertility and suitability for agriculture. These point data can be interpolated and what you get is a thematicmap showing isohyets or contour lines of rainfall.

Presenting Results

One of the most exciting aspects of GIS technology is the variety of different ways in which theinformation can be presented once it has been processed by GIS. Traditional methods of tabulating andgraphing data can be supplemented by maps and three dimensional images. Visual communication isone of the most fascinating aspects of GIS technology and is available in a diverse range of outputoptions.

Data Capture an Introduction


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The functionality of GIS relies on the quality of data available, which, in most developing countries, iseither redundant or inaccurate. Although GIS are being used widely, effective and efficient means of datacollection have yet to be systematically established. The true value of GIS can only be realized if theproper tools to collect spatial data and integrate them with attribute data are available.

Manual Digitization

Manual Digitizing still is the most common method for entering maps into GIS. The map to be digitized isaffixed to a digitizing table, and a pointing device (called the digitizing cursor or mouse) is used to tracethe features of the map. These features can be boundary lines between mapping units, other linearfeatures (rivers, roads, etc.) or point features (sampling points, rainfall stations, etc.) The digitizing tableelectronically encodes the position of the cursor with the precision of a fraction of a millimeter. The mostcommon digitizing table uses a fine grid of wires, embedded in the table. The vertical wires will record theY-coordinates, and the horizontal ones, the X-coordinates.

The range of digitized coordinates depends upon the density of the wires (called digitizing resolution) andthe settings of the digitizing software. A digitizing table is normally a rectangular area in the middle,separated from the outer boundary of the table by a small rim. Outside of this so-called active area of thedigitizing table, no coordinates are recorded. The lower left corner of the active area will have the

coordinates x = 0 and y = 0. Therefore, make sure that the (part of the) map that you want to digitize isalways fixed within the active area.

Scanning System

The second method of obtaining vector data is with the use of scanners. Scanning (or scan digitizing)provides a quicker means of data entry than manual digitizing. In scanning, a digital image of the map isproduced by moving an electronic detector across the map surface. The output of a scanner is a digitalraster image, consisting of a large number of individual cells ordered in rows and columns. For theConversion to vector format, two types of raster image can be used.

In the case of Chloropleth maps or thematic maps, such as geological maps, the individual mappingunits can be separated by the scanner according to their different colours or grey tones. The resulting

images will be in colours or grey tone images.

In the case of scanned line maps, such as topographic maps, the result is a black-and-white image.Black lines are converted to a value of 1, and the white areas in between lines will obtain a value of 0 inthe scanned image. These images, with only two possibilities (1 or 0) are also called binary images.

The raster image is processed by a computer to improve the image quality and is then edited andchecked by an operator. It is then converted into vector format by special computer programmes, whichare different for colour/grey tone images and binary images.

Scanning works best with maps that are very clean, simple, relate to one feature only, and do not containextraneous information, such as text or graphic symbols. For example, a contour map should only containthe contour line, without height indication, drainage network, or infrastructure. In most cases, such maps

will not be available, and should be drawn especially for the purpose of scanning. Scanning andconversion to vector is therefore, only beneficial in large organizations, where a large number of complexmaps are entered. In most cases, however, manual digitizing will be the only useful method for enteringspatial data in vector format.


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Figure 3

Data Conversion

While manipulating and analyzing data, the same format should be used for all data. This ScanningSystem implies that, when different layers are to be used simultaneously, they should all be in vector orall in raster format. Usually the conversion is from vector to raster, because the biggest part of theanalysis is done in the raster domain. Vector data are transformed to raster data by overlaying a grid witha user-defined cell size.

Sometimes the data in the raster format are converted into vector format. This is the case especially ifone wants to achieve data reduction because the data storage needed for raster data is much larger thanfor vector data.

A digital data file with spatial and attribute data might already exist in some way or another. There mightbe a national database or specific databases from ministries, projects, or companies. In some cases a

conversion is necessary before these data can be downloaded into the desired database.

The commonly used attribute databases are dBase and Oracle. Sometimes spreadsheet programmeslike Lotus, Quattro, or Excel are used, although these cannot be regarded as real database softwares.

Remote-sensing images are digital datasets recorded by satellite operating agencies and stored in theirown image database. They usually have to be converted into the format of the spatial (raster) databasebefore they can be downloaded.


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Spatial Data Management

Geo-Relational Data ModelAll spatial data files will be geo-referenced. Geo-referencing refers to the location of a layer or coveragein space defined by the coordinate referencing system. The geo relational approach involves abstractinggeographic information into a series of independent layers or coverages, each representing a selected setof closely associated geographic features (e.g., roads, land use, r iver, settlement, etc). Each layer has thetheme of a geographic feature and the database is organized in the thematic layers.

With this approach users can combine simple feature sets representing complex relationships in the realworld. This approach borrows heavily on the concepts of relational DBMS, and it is typically closelyintegrated with such systems. This is fundamental to database organization in GIS.

Topological Data Structure.Topology is the spatial relationship between connecting and adjacent coverage features (e.g., arc, nodes,polygons, and points). For instance, the topology of an arc includes from and to nodes (beginning of thearc and ending of the arc representing direction) and its left and right polygon. Topological relationshipsare built from simple elements into complex elements: points (simplest elements), arcs (sets of connectedpoints), and areas (sets of connected arcs). Topological data structure, in fact, adds intelligence to the

GIS database.

Attribute Data ManagementAll Data within a GIS (spatial data as well as attribute data) are stored within databases. A database is acollection of information about things and their relationships to each other. For example, you can have anengineering geological database, containing information about soil and rock types, field observations andmeasurements, and laboratory results. This is interesting data, but not very useful if the laboratory data,for example, cannot be related to soil and rock types.

The objective of collecting and maintaining information in a database is to relate facts and situations thatwere previously separate.

The principle characteristics of a DBMS are: -

Centralized control over the database is possible, allowing for better quality management and operator-defined access to parts of the database;

Data can be shared effectively by different applications;

The access to the data is much easier, due to the use of a user-interface and the user-views (especiallydesigned formula for entering and consulting the database);

Data redundancy (storage of the same data in more than one place in the database) can be avoided asmuch as possible; redundancy or unnecessary duplication of data are an annoyance, since this makesupdating the database much more difficult; one can easily overlook changing redundant informationwhenever it occurs; and

The creation of new applications is much easier with DBMS.

The disadvantages relate to the higher cost of purchasing the software, the increased complexity ofmanagement, and the higher risk, as data are centrally managed.

Relational Database -- Concepts & ModelThe relational data model is conceived as a series of tables, with no hierarchy nor any predefinedrelations. The relation between the various tables should be made by the user. This is done by identifyinga common field in two tables, which is assigned as the flexibility than in the other two data models.


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However, accessing the database is slower than with the other two models. Due to its greater flexibility,the relational data model is used by nearly all GIS systems

Choosing geographic dataThe main purpose of purchasing a geographic information system (GIS)* is to produce results for yourorganization. Choosing the right GIS/mapping data will help you produce those results effectively.

The role of base-map data in your GIS,

The common characteristics of geographic data,

The commonly available data sources

Guidelines for evaluating the suitability of any data set for your project.

The world of GIS data is complex, by choosing the right data set, you can save significant amounts ofmoney and, even more importantly, quickly begin your GIS project.

Data: The Core of Your Mapping / GIS Project

When most people begin a GIS project, their immediate concern is with purchasing computer hardware

and software. They enter into lengthy discussions with vendors about the merits of various componentsand carefully budget for acquisitions. Yet they often give little thought to the core of the system, the datathat goes inside it. They fail to recognize that the choice of an initial data set has a tremendous influenceon the ultimate success of their GIS project.

Data, the core of any GIS project, must be accurate - but accuracy is not enough. Having the appropriatelevel of accuracy is vital. Since an increase in data accuracy increases acquisition and maintenancecosts, data that is too detailed for your needs can hurt a project just as surely as inaccurate data can. Allany GIS project needs is data accurate enough to accomplish its objectives and no more. For example,you would not purchase an engineering workstation to run a simple word-processing application.Similarly, you would not need third-order survey accuracy for a GIS-based population study whosesmallest unit of measurement is a county. Purchasing such data would be too costly and inappropriate forthe project at hand. Even more critically, collecting overly complex data could be so time-consuming that

the GIS project might lose support within the organization.

Even so, many people argue that, since GIS data can far outlast the hardware and software on which itruns, no expense should be spared in its creation. Perfection, however, is relative. Projects and datarequirements evolve. Rather than overinvest in data, invest reasonably in a well-documented, well-understood data foundation that meets today's needs and provides a path for future enhancements. Thisapproach is a key to successful GIS project implementation.

Are Your Data Needs Simple or Complex?Before you start your project, take some time to consider your objectives and your GIS data needs. Askyourself, "Are my data needs complex or simple?"

*Italicized words can be found in the Glossary at the end of this document except for words used for

emphasis or words italicized for reasons of copyediting convention or layout.If you just need a map as a backdrop for other information, your data requirements are simple. You arebuilding a map for your specific project, and you are primarily interested in displaying the necessaryinformation, not in the map itself. You do not need highly accurate measurements of distances or areas orto combine maps from different sources. Nor do you want to edit or add to the map's basic geographicinformation.

An example of simple data requirements is a map for a newspaper story that shows the location of a fire.Good presentation is important; absolute accuracy is not.


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If you have simple data needs, read this paper to get the overall picture of what GIS data is and how it fitsinto your project. A project with simple data requirements can be started with inexpensive maps. Yourprimary interests will be quality graphic- display characteristics and finding maps that are easy to use withyour software. You need not be as concerned with technical mapping issues. However, basic knowledgeof concepts such as coordinate systems, absolute accuracy, and file formats will help you understandyour choices and help you make informed decisions when it's time to add to your system.

What issues suggest more complex GIS data needs?

Building a GIS to be used by many people over a long period of time.

Storing and maintaining database information about geographic features.

Making accurate engineering measurements from the map.

Editing or adding to the map.

Combining a variety of information from different sources.

An example of a system requiring complex data would be a GIS built to manage infrastructure for anelectric utility.

If your data requirements are complex, you ought to pay particular attention to the sections of this paperthat discuss data accuracy, coordinate systems, layering, file formats, and the issues involved incombining data from different sources.

Also keep in mind that projects evolve, and simple data needs expand into complex ones as your projectmoves beyond its original objectives. If you understand the basics of your data set, you will make betterdecisions as your project grows.

Basics of Digital Mapping

Vector vs. Raster MapsThe most fundamental concept to grasp about any type of graphic data is making the distinction between

vector data and raster data. These two data types are as different as night and day, yet they can look thesame. For example, a question that commonly comes up is "How can I convert my TIFF files into DXFfiles?" The answer is "With difficulty," because TIFF is a raster data format and DXF (data interchangefile) is a vector format. And converting from raster to vector is not simple. Raster maps are best suited tosome applications while vector maps are suited to others.

Figure 4

Raster data represents a graphic object as a pattern of dots, whereas vector data represents the object


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as a set of lines drawn between specific points. Consider a line drawn diagonally on a piece of paper. Araster file would represent this image by subdividing the paper into a matrix of small rectangles-similar toa sheet of graph paper-called cells (figure 1). Each cell is assigned a position in the data file and given avalue based on the color at that position. White cells could be given the value 0; black cells, the value 1;grays would fall in-between. This data representation allows the user to easily reconstruct or visualize theoriginal image.

Figure 5

A vector representation of the same diagonal line would record the position of the line by simply recording

the coordinates of its starting and ending points. Each point would be expressed as two or three numbers(depending on whether the representation was 2D or 3D, often referred to as X,Y or X,Y,Z coordinates(figure 2). The first number, X, is the distance between the point and the left side of the paper; Y, thedistance between the point and the bottom of the paper; Z, the point's elevation above or below thepaper. The vector is formed by joining the measured points.

Some basic properties of raster and vector data are outlined below.

Each entity in a vector file appears as an individual data object. It is easy to record informationabout an object or to compute characteristics such as its exact length or surface area. It is muchharder to derive this kind of information from a raster file because raster files contain little (andsometimes no) geometric information.

Some applications can be handled much more easily with raster techniques than with vector

techniques. Raster works best for surface modeling and for applications where individual featuresare not important. For example, a raster surface model can be very useful for performing cut-and-fill analyses for road-building applications, but it doesn't tell you much about the characteristics ofthe road itself. Terrain elevations can be recorded in a raster format and used to construct digitalelevation models (DEMs) (figure 3). Some land-use information comes in raster format.

Figure 6


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Raster files are often larger than vector files. The raster representation of the line in the exampleabove required a data value for each cell on the page, whereas the vector representation onlyrequired the positions of two points.

The size of the cells in a raster file is an important factor. Smaller cells improve image quality because

they increase detail. As cell size increases, image definition decreases or blurs. In the example, theposition of the line's edge is defined most clearly if the cells are very small. However, there is a trade-off:Dividing the cell size in half increases file size by a factor of four.

Cell size in a raster file is referred to as resolution. For a given resolution value, the raster cost does notincrease with image complexity. That is, any scanner can quickly make a raster file. It takes no moreeffort to scan a map of a dense urban area than to scan a sparse rural one. On the other hand, a vectorfile requires careful measuring and recording of each point, so an urban map will be much more time-consuming to draw than a rural map. The process of making vector maps is not easily automated, andcost increases with map complexity.

Because raster data is often more repetitive and predictable, it can be compressed more easily thanvector data. Many raster formats, such as TIFF, have compression options that drastically reduce imagesizes, depending upon image complexity and variability.

Raster files are most often used:

For digital representations of aerial photographs, satellite images, scanned paper maps, andother applications with very detailed images.

When costs need to be kept down.

When the map does not require analysis of individual map features.

When "backdrop" maps are required.

In contrast, vector maps are appropriate for:

Highly precise applications.

When file sizes are important. When individual map features require analysis.

When descriptive information must be stored.

Raster and vector maps can also be combined visually. For example, a vector street map could beoverlaid on a raster aerial photograph. The vector map would provide discrete information aboutindividual street segments, the raster image, a backdrop of the surrounding environment.

Digital Map Formats- How Data Is StoredThe term file format refers to the logical structure used to store information in a GIS file. File formats areimportant in part because not every GIS software package supports all formats. If you want to use a dataset, but it isn't available in a format that your GIS supports, you will have to find a way to transform it, find

another data set, or find another GIS.

Almost every GIS has its own internal file format. These formats are designed for optimal use inside thesoftware and are often proprietary. They are not designed for use outside their native systems. Mostsystems also support transfer file formats. Transfer formats are designed to bring data in and out of theGIS software, so they are usually standardized and well documented.

If your data needs are simple, your main concern will be with the internal format that your GIS softwaresupports. If you have complex data needs, you will want to learn about a wider range of transfer formats,


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especially if you want to mix data from different sources. Transfer formats will be required to import somedata sets into your software.

Vector FormatsMany GIS applications are based on vector technology, so vector formats are the most common. Theyare also the most complex because there are many ways to store coordinates, attributes, attributelinkages, database structures, and display information. Some of the most common formats are brieflydescribed below

Common Vector File Formats

Format NameSoftwarePlatform

Internal orTransfer

Developer Comments

Arc Export ARC/INFO* TransferEnvironmentalSystems ResearchInstitute, Inc. (ESRI)

Transfers dataacross ARC/INFO*platforms.

ARC/INFO* Coverages ARC/INFO* Internal ESRI

AutoCAD Drawing Files

(DWG)

AutoCAD* Internal Autodesk

Autodesk DataInterchange File (DXF)

Many Transfer AutodeskWidely used graphicstransfer standard.

Digital Line graphs (DLG) Many TransferUnited StatesGeological Survey(USGS)

Used to publishUSGS digital maps.

Hewlett-Packard GraphicLanguage (HPGL)

Many Internal Hewlett-PackardUsed to control HPplotters.

MapInfo Data TransferFiles (MIF/MID)

MapInfo* Transfer MapInfo Corp.

MapInfo Map Files MapInfo* Internal MapInfo Corp.

MicroStation Design Files(DGN)

MicroStation* Internal Bentley Systems, Inc.

Spatial Data TransferSystem (SDTS)

Many (in thefuture)

Transfer US GovernmentNew US standard forvector and rastergeographic data.

Topologically IntegratedGeographic Encoding andReferencing (TIGER)

Many Transfer US Census BureauUsed to publish USCensus Bureaumaps.

Vector Product Format(VPF)

Militarymappingsystems

BothUS Defense MappingAgency

Used to publishDigital Chart of theWorld.

Raster FormatsRaster files generally are used to store image information, such as scanned paper maps or aerialphotographs. They are also used for data captured by satellite and other airborne imaging systems.Images from these systems are often referred to as remote-sensing data. Unlike other raster files, whichexpress resolution in terms of cell size and dots per inch (dpi), resolution in remotely sensed images isexpressed in meters, which indicates the size of the ground area covered by each cell.


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Some common raster formats are described below

Format NameSoftwarePlatform

Internal orTransfer

Developer Comments

Arc DigitizedRaster Graphics

(ADRG)

Militarymapping

systems

BothUS DefenseMapping Agency

Band Interleavedby Line (BIL)

Man BothCommon remote-sensing standard.

Band Interleavedby Pixel (BIP)

Many BothCommon remote-sensing standard.

Band Sequential(BSQ)

Many BothCommon remote-sensing standard.

Digital ElevationModel for (DEM)

Many TransferUnited StatesGeological Survey(USGS)

USGS standard format digitalterrain models.

PC Paintbrush

Exchange (PCX) PC Paintbrush Both Zsoft Widely used raster format.

Spatial DataTransfer Standard(SDTS)

Many (in thefuture)

TransferUS FederalGovernment

New US standard for both rasterand vector geographic data; rasterversion still under development.

Tagged ImageFile Format(TIFF)

PageMaker Both Aldus Widely used raster format.

An Example of Raster and Vector Integration


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Figure 7: An Example of Raster and Vector Integration

Vectors & Raster Data Models - Merits & Demerits.

RASTER MODEL VECTOR MODEL

Advantages

Simple data structure

Easy and efficient overlaying

Compatible with RS imagery

High spatial variability is efficientlyrepresented

Simple for own programming

Same grid cells for several attributes

Disadvantages

Inefficient use of computer storage

Errors in perimeter, and shape

Difficult network analysis Inefficient projection transformations

Loss of information when using large cellsLess accurate (although interactive) maps

Advantages

Compact data structure

Efficient for network analysis

Efficient projection transformation

Accurate map output.

Disadvantages

Complex data structure

Difficult overlay operations

High spatial variability is inefficientlyrepresented

Not compatible with RS imagery

Hybrid System


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It is an integration of the best of Vector and Raster Models. The GIS technology is fast moving towardsHybrid model GIS.

The Integration of Vector and Raster System Hybird System

Figure 8: The Integration of Vector and Raster System Hybird System


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Analysis of Geographic Data

ANALYSIS - What? & Why?The heart of GIS is the analytical capabilities of the system. What distinguish the GIS system from otherinformation system are its spatial analysis functions. Although the data input is, in general, the most timeconsuming part, it is for data analysis that GIS is used. The analysis functions use the spatial and non-

spatial attributes in the database to answer questions about the real world. Geographic analysis facilitatesthe study of real-world processes by developing and applying models. Such models illuminate theunderlying trends in geographic data and thus make new information available. Results of geographicanalysis can be communicated with the help of maps, or both.

The organization of database into map layers is not simply for reasons of organizational clarity, rather it isto provide rapid access to data elements required for geographic analysis. The objective of geographicanalysis is to transform data into useful information to satisfy the requirements or objectives of decision-makers at all levels in terms of detail. An important use of the analysis is the possibility of predictingevents in the another location or at another point in time.

ANALYSIS - How?Before commencing geographic analysis, one needs to assess the problem and establish an objective.The analysis requires step-by-step procedures to arrive at the conclusions.

The range of geographical analysis procedures can be subdivided into the following categories.

Database Query.

Overlay.

Proximity analysis.

Network analysis.

Digital Terrain Model.

Statistical and Tabular Analysis.

Spatial AnalysisIt helps us to:

Identify trends on the data.

Create new relationships from the data.

View complex relationships between data sets.

Make better decisions.

Geographic AnalysisAnalysis of problems with some Geographic Aspects.

Alternatives are geographic locations or areas.

Decisions would affect locations or areas.

Geographic relationships are important in decision-making or modelling.

Some examples of its application:

Nearest Neighbour.

Network distances.

Planar distances.

Spatial Analysis - An Application


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Image 1

Where should we build a road from a point A to point B?

How do we minimise the impacts of building this road?

Relationship of Modelling to Analysis

Decision Models search through potential alternatives to arrive at a recommendation. Decision support models process raw data into forms that are directly relevant to decision

making.

Data characterisation models are used to develop a better understanding of a system to helpcharacterise a problem or potential solutions.

Difficulties of Geographic Analysis

Plenty of data.

Spatial relationships are important but difficult to measure.

Inherent uncertainty due to scale.

any data sources.

Difficult to make data sources compatible. Difficult mathematics.

Quantity vs. Quality Questions.

Multiple objectives.

GIS can address some (but not all) of these difficulties.

Network Analysis


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Network models are based on interconnecting logical components, of which the most important are:

1. "Nodes" define start, end, and intersections2. "Chains" are line features joining nodes3. "Links" join together points making up a chain.

This network can be analyzed using GIS.A simple and most apparent network analysis applications are:

Street network analysis,

Traffic flow modelling,

Telephone cable networking,

Pipelines etc.

The other obvious applications would be service centre locations based on travel distance.

Basic forms of network analysis simply extract information from a network. More complex analysis,process information in the network model to derive new information. One example of this is the classicshortest-path between two points. The vector mode is more suited to network analysis than the raster

model.

A Road Network

Image

Tabular Statistical Analysis

If in the above road network we have categorised the streets then in such a case the statistical analysisanswers questions like

What unique categories do I have for streets?

How many features do I have for each unique category?

Summarize by using any attribute?

Database Query


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The selective display and retrieval of information from a database are among the fundamentalrequirements of GIS. The ability to selectively retrieve information from GIS is an important facility.Database query simply asks to see already stored information. Basically there are two types of querymost general GIS allow: viz.,

Query by attribute,Query by geometry.

Map features can be retrieved on the basis of attributes, For example, show all the urban areas havingthe population density greater than 1,000 per square kilometer, Many GIS include a sophisticated functionof RDBMS known as Standard Query Language (SQL), to search a GIS database. The attributedatabase, in general, is stored in a table (relational database mode.) with a unique code linked to thegeometric data. This database can be searched with specific characteristics. However, more complexqueries can be made with the help of SQL.

GIS can carry out a number of geometric queries. The simplest application, for example, is to show theattributes of displayed objects by identifying them with a graphical cursor. There are five forms of primitivegeometric query: viz.,

Query by point,

Query by rectangle,Query by circle,Query by line,Query by polygon,

A more complex query still is one that uses both geometric and attributes search criteria together. ManyGIS force the separation of the two different types of query. However, some GIS, using databases tostore both geometric and attribute data, allow true hybrid spatial queries.Overlay OperationsThe hallmark of GIS is overlay operations. Using these operations, new spatial elements are created bythe overlaying of maps.There are basically two different types of overlay operations depending upon data structures:

Raster overlayIt is a relatively straightforward operation and often many data sets can be combined anddisplayed at once.

Vector overlayThe vector overlay, however is far more difficult and complex and involves moreprocessing.

Logical OperatorsThe concept of map logic can be applied during overlay. The logical operators are Boolean functions.There are basically four types of Boolean Operators: viz., OR, AND, NOT, and XOR.

With the use of logical, or Boolean, operators spatial elements / or attributes are selected that fulfillcertain condition, depending on two or more spatial elements or attributes.

Vector OverlayDuring vector overlay, map features and the associated attributes are integrated to produce newcomposite maps. Logical rules can be applied to how the maps are combined. Vector overlay can beperformed on different types of map features: viz.,

Polygon-on-polygon overlay

Line-in-polygon overlayPoint-on-polygon overlay


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During the process of overlay, the attribute data associated with each feature type id merged. Theresulting table will contain both the attribute data. The process of overlay will depend upon the modellingapproach the user needs. One might need to carry out a series of overlay procedures to arrive at theconclusion, which depends upon the criterion.

Polygon-on-Polygon Overlay

Polygon-on-Polygon Overlay

Difference between a Topologic Overlay and a Graphic Over plot


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Difference between a Topologic Overlay and a Graphic Over plot

Raster OverlayIn raster overlay, the pixel or grid cell values in each map are combined using arithmetic and Booleanoperators to produce a new value in the composite map. The maps can be treated as arithmeticalvariables and perform complex algebraic functions. The method is often described as map algebra. Theraster GIS provides the ability to perform map layers mathematically. This is particularly important for themodelling in which various maps are combined using various mathematical functions. Conditionaloperators are the basic mathematical functions that are supported in GIS.

Conditional OperatorsConditional operators were already used in the examples given above. The all evaluate whether a certaincondition has been met.

= eq 'equal' operator ne 'non-equal' operator< lt 'less than' operator gt 'greater than' operator>= ge 'greater than or equal' operator

Many systems now can handle both vector and raster data. The vector maps can be easily draped on tothe raster maps.


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Raster Overlay

Raster Overlay

Buffer OperationUsing these operations, the characteristics of an area surrounding in a specified location are evaluated.This kind of analysis is called proximity analysis and is used whenever analysis is required to identifysurrounding geographic features. The buffer operation will generate polygon feature types irrespective ofgeographic features and delineates spatial proximity. For example, what are the effects on urban areas ifthe road is expanded by a hundred meters to delineate a five-kilometer buffer zone around the nationalpark to protect it from grazing.


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Using Buffer

Using Buffer

Digital Terrain ModelThe object of Digital Terrain analysis is to represent a surface and its properties accurately. This isnormally achieved by creating a digital terrain model, often known as DTM, formed by sampling thesurface. A digital terrain model can be viewed in two different ways:

as an isoline map,

as an isometric model.

Isolines join points of equal value on a surface. The shading defines bands, including all heights, betweenthe isolines.

Isometric models can be shown in three-dimensional models. These models show the terrain inperspective so that the apparent height is proportional to the value of the point. Visualisation techniquesare used to project the model from the given eyepoint.

Spatial Analysis - a Process


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Spatial Analysis a Process


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Projection System

Maps are flat, but the surfaces they represent are curved. Transforming, three-dimensional space onto atwo dimensional map is called "projection". This process inevitably distorts at least one of the followingproperties:

Shape, Area,

Distance,

Direction, and often more.

It is known that a globe is a true representation of the earth, which is divided into various sectors by thelines of latitudes and longitudes. This network is called 'graticule'. A map projection denotes thepreparation of the graticule on a flat surface.

Theoretically map projection might be defined as "a systematic drawing of parallels of latitude andmeridians of longitudes on a plane surface for the whole earth or a part of it on a certain scale so that anypoint on the earth surface may correspond to that on the drawing."

Necessity of Map ProjectionAn ordinary globe is rendered useless for reference to a small country. It is not possible to make a globeon a very large scale. Say, if anyone wants to make a globe on a scale of one inch to a mile, the radiuswill be 330 ft. It is difficult to make and handle such a globe and uncomfortable to carry it in the field forreference. Not only topographical maps of different scales but also atlas and wall maps would not havebeen possibly made without the use of certain projections. So a globe is least useful or helpful in the fieldof practical purposes. Moreover it is neither easy to compare different regions over the globe in detail, norconvenient to measure distances over it. Therefore for different types of maps different projections havebeen evolved in accordance with the scale and purpose of the map.

Selection of Map ProjectionThere is no ideal map projection, but representation for a given purpose can be achieved. The selectionof projection is made on the basis of the following:

The location and the extension of the feature of the globe.

1. The shape of the boundary to be projected.2. The deformations or distortions of a map to be minimized.3. The mathematical model to be applied to preserve some identity of graphical features.

Based on these characteristics the utility of the projection is ascertained.

Some Interesting Links :

Map Projection OverviewAn Article by Peter H. Dana

Map ProjectionsAn Article by Brian Klinkenberg

Map ProjectionBy Worlfram Research

Map Projection TutorialWhy are map projections an issue in GIS? - British Columbia

Map ProjectionTutorial on Map Projection by Peter H. Dana


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Map ProjectionLearn about Map Projection from United States Environmental Protection Agency

ClassificationPotentially there exits an unlimited number of map projections possessing one property or the other. Thenatures of these properties are so complex that they often possess one or more common properties.

There is no projection, which can be grouped, in a single class. Moreover, if one attempts to obtain arational classification of map projection, it will be rather difficult to achieve it. There can be as manyclassifications as many bases.

Depending on different bases the following classifications may be suggested:

Basis Classes

1. Method of Construction 1. Perspective2. Non-perspective

2. Preserved qualities 1. Homolographic / Equal Area2. Orthomorphic / Conformal

3. Developable surface area 1. Cylindrical2. Conical3. Azimuthal / Zenithal

4. Conventional4. Position of tangent surface 1. Polar

2. Equation/Normal3. Oblique

5. Position of viewpoint or light 1. Gnomonic2. Stereographic3. Orthographic4. Others

Classification based on methods of constructionMathematically the term 'projection' means the determination of points on the plane as viewed from afixed point. But in cartography it may not be necessarily restricted to 'perspective' or geometricalprojection. On the globe the meridians and parallels are circles. When they are transferred on a plane

surface, they become intersecting lines, curved or straight. If we stick a flat paper over the globe, it willnot coincide with it over a large surface without being creased. The paper will touch the globe only at onepoint, so that the other sectors will be projected over plane in a distorted form. The projection with thehelp of light will give a shadowed picture of the globe which is distorted in those parts which are fartherfrom the point where the paper touches it. The amount of distortion increases with the increase indistance from the tangential point. But only a few of the projections imply this perspective method.

The majority of projections represent an arrangement of lines of latitude and longitude in conformity withsome principles so as to minimize the amount of distortion. With the help of mathematical calculationstrue relation between latitude and longitudes is maintained. Thus various processes of non-perspectiveprojections have been devised.


Classification of Map ProjectionAn Article from University of Waterloo

Classification based on preserved qualitiesWhile transferring the globe on a plane surface some facts should be kept in view:

1. Preservation of area,2. Preservation of shape,


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3. Preservation of bearing i.e. direction and distance.

It is, however, very difficult to make such a projection even for a small country, in which all the abovequalities may be well preserved. Any one quality may be thoroughly achieved by a certain map projectiononly at the cost of others.

According to the quality they preserve, projections may be classified into three groups :-

1. Equal area (Homolographic projection),2. Correct shape (Orthomorphic or Conformal projection),3. True bearing (Azimuthal projection).

Classification based on developable surface areaThere are some surfaces over which the sphere may be projected. After projection such surfaces may becut open onto flat surface. These developable surfaces include

1. Cylinder and2. Cone.

Cylindrical ProjectionWhen the graticule is prepared on the surface of a hollow cylinder it is called Cylindrical Projection.

1. Normal Cylindrical Projection - This is a perspective cylindrical projection. When a cylinder iswrapped round the globe so as to touch it along the equator, and the light is placed at the centre,the true cylindrical projection is obtained.

Limitations:The scale is true only along the equator. The exaggeration of the parallel scale as well as themeridian scale would be very greatly increasing away from the equator. The poles can't beshown, because their distances from the equator becomes infinite.

2. Simple Cylindrical Projection - It is also called Equidistant Cylindrical Projection as both theparallels and meridians are equidistant. The whole network represents a series of equal squares.All the parallels are equal to the equator and all the meridians are half of the equator in length.The projection is neither equal area nor orthomorphic.

Limitations :The scale along the equator is true. The meridian scale is correct everywhere because theparallels are drawn at their true distances. Latitudinal scale increases away from the equator.This leads to great distortion in shape and exaggeration of area in high latitudes.

3. Cylindrical Equal Area Projection - This cylindrical projection was introduced by Lambert. Theproperties are almost the same. The area between two parallels is made equal to thecorresponding surface on the sphere at the cost of great distortion in shape towards higher

latitudes; this is why it is an equal area projection.

Limitations: Same as (ii).

General Properties of Cylindrical Projection

Cylindricals are true at the equator and the distortion increases as on moves towards the poles.

Good for areas in the tropics


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Conical ProjectionA cone may be imagined to touch the globe of a convenient size along any circle (other than a greatcircle) but the most useful case will be the normal one in which the apex of the cone will lie verticallyabove the pole on the earth's axis produced and the surface of the cone will be tangent to the spherealong some parallel of latitude. It is called 'standard parallel'.

If the selected parallel (SP) is nearer the pole the vertex of the cone will be closer to it and subsequentlythe angle at the apex will be increasing proportionately. When the pole itself becomes the selectedparallel, the angle of the apex will become 180 degrees, and the surface of the cone will be similar to thetangent plane of Zenithal Projection.

On the other hand, when the selected parallel is nearer to the equator, the vertex of the cone will bemoving farther away from the pole. in case equator is the selected parallel, the vertex will be at an infinitedistance, and the cone will become a cylinder.Thus the Cylindrical and Zenithal Projections may be regarded as special cases of Conical Projections.

Properties

Conics are true along some parallel somewhere between the equator and the pole and thedistortion increases away from this standard.

Good for Temperate Zone areas

Zenithal ProjectionIn Zenithal Projection a flat paper is supposed to touch the globe at one point and the light may be kept atanother point so as to reflect or project the lines of latitude and longitude on the plane. Here the globe isviewed from a point vertically above it, so these are called Zenithal Projections. They are also called'azimuthal' because the bearings are all true from the central point.

In respect of the plane's position touching the globe, Zenithal Projection is of three main classes :-

1. Normal or Equatorial Zenithal (where the plane touches the globe at equator),2. Polar Zenithal (where the plane touches the globe at pole),

3. Oblique Zenithal (where the plane touches the globe at any other point).

According to the location of the view point Zenithal Projection is of three types :-

1. Gnomonic / Central (view point lies at the centre of the globe),2. Stereographic (view point lies at the opposite pole)3. Orthographic (view point lies at the infinity).

Properties

Azimuthals are true only at their centre point, but generally distortion is worst at the edge of themap.

Good for polar areas.


Projection Classification by 'Developable Surface'By Cartography - Spatial Data Techniques

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